The Royal Exchange's Truth About Youth festival claims to be about "challenging and changing negative perceptions of young people in the UK". On the face of it, inviting a group of teenagers to come and trash the auditorium might seem a curious way to go about it. In mitigation, though, they bring their grandparents with them, who prove to be equally destructive.

The piece begins, innocuously enough, like a slightly awkward group-therapy session. Participants ranging in age from 13 to 78 introduce themselves and summarise what they most want from life. "How do you know when you've become old?" one of the youngsters asks. "When you start getting junk mail from Saga," comes the reply.

Then something rather peculiar begins to happen. The participants busy themselves setting up the accoutrements of a garden party – fake turf, fairy lights, petits fours. A rock band turns up and plugs in.

The questions become less comfortable ("When and where would you like to die?" "Will my pension last throughout my retirement?" "Will there still be pensions when I'm old?"). The atmosphere turns lairy; a jug of water is spilled. Before you know it, a gentle, well-intentioned community project self-combusts in a melee of flying cake stands and plastic chairs. The implication seems to be that, no matter how old you are, everyone shares the fear of unforeseen disaster.

But although Max Webster's direction gives the narrative a compelling framework, the editorial hand of writer Sonia Hughes is largely invisible. "Do you ever have those dreams where you wake up feeling as if you've been sobbing all night?" someone asks. A dream that feels rather like this surreal, wildly unpredictable and curiously heartbreaking little show.